drivers/clk/clk_zynqmp.c: get rid of compiler warning for !CONFIG_CMD_CLK builds

drivers/clk/clk_zynqmp.c: get rid of compiler warning for !CONFIG_CMD_CLK builds

When built without CONFIG_CMD_CLK, we get a warning about the unused
clk_names variable:

../drivers/clk/clk_zynqmp.c:153:27: warning: ‘clk_names’ defined but not used [-Wunused-const-variable=]
  153 | static const char * const clk_names[clk_max] = {

So also guard it with CONFIG_CMD_CLK to get rid of that.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20260119095437.2775081-2-peter@korsgaard.com